The term substrate gets used in two senses. In the first place, it can refer to the substance that the fungus uses as food. The mycelium of the fungus grows through this kind of substrate, secreting enzymes that dissolve part of the substrate, and absorbing the nutrition that results. This "growing through" leads us to the second sense, which is "whatever sort of substance the fruiting body (or other visible token of the fungus) appears on." Usually, these two senses refer to the same substance, but not always.

Sometimes the wood that a fungus is eating is buried (as with the roots of a stump, or the remains of a stump that has been only partially removed), so the fruiting bodies have to come up through the ground. We sometimes say that such a mushroom is "apparently terrestrial," but it may also just get called " terrestrial when growing on buried wood."

Second, saxicolouslichens grow on rocks, and even attack the rock by secreting acids that dissolve the rock (without dissolving themselves; neat trick!), but the rock is generally not considered a serious source of nutrition for the lichen. Probably it is just roughening the rock surface, or making crevices in the rock, to make it easier for the lichen to hold on.